1. Field of the Invention
The invention relates to a knee prosthesis.
2. Description of Related Art
The knee joint or articulation may be considered two condyloid joints, lateral and medial, between femur and tibia, and one arthrodial joint between the patella and the femur. The chief movements at the knee are flexion (decrease in the angle between two bones) and extension (increase in the angle between two bones) and rotation. These movements can be referred to as asymmetrical in that the movement of the left knee joint differs from the movement of the right knee joint. The individual displacement of the right and left knee joint during flexion and extension is also asymmetrical.
The knee joint combines a wide range of movement in one direction with a great weight-bearing capacity and considerable stability. The superior end of the tibia is the largest weight-bearing surface of the human skeleton. Its two articulating condyles or menisci are thickened and convex on their peripheral borders, and thin, concave, and free on their opposite borders. They are connected anteriorly and peripherally by transverse ligaments, and by part of the capsule of the knee joint, to the head of the tibia. These menisci lend some stability to the joint. Additional stability is given by the strong anterior and posterior cruciate ligaments which connect the tibia and femur inside of the joint and cross each other like the letter "X". The anterior cruciate ligament extends from the front of the intercondylar eminence of the tibia, upward and backward to the medial side of the lateral condyle of the femur. The posterior cruciate ligament extends from the posterior intercondylar fossa of the tibia, upward and forward to the lateral side of the medial condyle of the femur. The stability of the knee is secured by the muscles of the thigh, the joint capsule (system of tendons and ligaments that pass over the knee joint) and four ligaments--the two lateral ligaments and two cruciate ligaments.
Injuries to the knee are very common. The injuries often result to the menisci or the ligaments that hold them. Significant research and development in recent years has been directed to the development of knee prostheses that are reliable, i.e., prostheses that are not subject to unacceptable dislocation, not subject to bearing failure, not subject to loosening from the bones, and which provide a substantial duplication of the motion of the natural joint. In general, knee replacement prostheses are indicated for bi-cruciate retention application, unicondylar applications and for posterior cruciate retention applications. Other prostheses are indicated where neither posterior nor anterior cruciate ligaments are retained. The types of knee prostheses available can generally be classified as fixed prostheses and mobile prostheses.
Generally, either a fixed or mobile knee prosthesis involves a femoral component, a meniscal component, and a tibial component. The meniscal component generally is seated between the femoral component and the tibial component, each mated with the femur and tibia, respectively. The reference to either fixed or mobile prostheses generally concerns the meniscal component. In the fixed system, the meniscal component is fixedly attached to the femur or tibia. In the mobile system, prior art knee prostheses offer some limited range of symmetrical motion for each of the right and left knee joint prosthesis.
The fixed prosthesis is generally used on patients where there is severe damage to the femur and/or tibia around the knee joint or where neither the posterior or anterior cruciate ligaments of the knee joint cannot be retained. The fixed prosthesis generally does not allow any movement of the motion of the femur on the tibia, e.g., the "sliding-rolling" motion of the femur on the tibia. Instead, the meniscal component is fixed to the tibial component and/or the femoral component. This fixation generally includes screw and bands. The fixed prosthesis also does not allow correction for a misplacement in rotation of the tibia component. Finally, the fixed prosthesis contributes to accelerated wear of the generally polyethylene meniscal component.
Mobile, i.e., sliding or moving, knee prostheses generally accommodate some movement by the meniscal component or the tibial component during knee joint movement. As noted above, the individual biomechanical displacement of the right and left knee joint during flexion and extension is asymmetrical. The natural meniscal displacement of a knee joint during extension, for example, is approximately 15 millimeter (mm) for the external (lateral) meniscus and 5 mm for the internal (medial) meniscus.
The general interest in the mobile prosthesis is to obtain a dimunition of the constraint on the meniscal component by delivering a proper positioning of the meniscal component on the tibial component during and after movement. In most instances, the motions of prior art prostheses are limited to a simple rotation (flexion/extension) which is in some instances combined with anterior-posterior clearance. These protheses generally offer no lateral translation or anterior-posterior translation of the components, e.g., the meniscal component. The range of motion of the components for the displacement is limited generally because the guidance is accomplished on rails or the motion around a fixed axis. The existing mobile motions are also symmetrical and non-conforming to human biomechanical movements. For example, most mobile knee prostheses have an axis of rotation about which movements of flexion and extension take lace. In these systems, the displacement of the meniscal component about the axis of rotation is symmetrical. For example, displacement of the external (lateral) portion of the meniscal component is equivalent to the displacement of the internal (medial) portion for extension and flexion. During flexion, this type of symmetrical displacement will cause the femoral component to strike and erode the internal meniscal component and reduce flexion.
Prior art mobile prostheses also offer no rotational misalignment correction, such as, for example, where the meniscal component is misaligned between the femoral and tibial components. This is especially true in those systems that provide guide rails in the seat of the tibial component for placement of the meniscal component. The mobile prostheses further provide a lack of simple transformation toward a fixed tibial plate in cases of lateral instability, risk of incorrect positioning, luxation of the meniscal component, and rupture of the posterior cruciate ligament. In this instance, additional surgery is necessary to place a fixed knee prosthesis.
The invention seeks to address the limitations inherent in prior art knee prostheses.